1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly, to a reflective liquid crystal display device and a fabricating method thereof. Although the present invention is suitable for a wide scope of applications, it is particularly suitable for providing a reflective liquid crystal display device with a high aperture ratio and a high brightness.
2. Discussion of the Related Art
In general, liquid crystal display (LCD) devices are classified into two categories according to a method of using a light source: transmissive LCD devices using a backlight unit and reflective LCD devices using an external light source. The transmissive LCD devices use a backlight unit, which consumes more than two thirds of the total power. On the other hand, since the reflective LCD devices use an external light source instead of a backlight unit, power consumption is reduced. However, the reflective LCD devices have low contrast ratio and low brightness because of the absence of a bright light source such as the backlight unit. To improve contrast ratio, a black matrix is generally used for the reflective LCD devices. However, the black matrix reduces a reflection area, thereby brightness is reduced.
FIG. 1 is a schematic perspective view of a reflective liquid crystal display device according to a related art.
In FIG. 1, first and second substrates 6 and 23 face into and are spaced apart from each other. A gate line 5 and a data line 17 crossing each other are formed on an inner surface of the first substrate 6. The gate line 5 and the data line 17 define a pixel region “P”. A thin film transistor (TFT) “T” adjacent to each intersection of the gate line 5 and the data line 17 is connected to the gate line 5 and the data line 17. A reflective electrode (a pixel electrode) 18 connected to the TFT “T” is formed in the pixel region “P”. The reflective electrode 18 may be made of a conductive material having high reflectance. For example, aluminum (Al) or Al alloy can be used for the reflective electrode 18.
A black matrix 21 and a color filter layer 22 including red, green, and blue sub-color filters 22a, 22b, and 22c are formed on an inner surface of the second substrate 23. Each sub-color filter 22a, 22b, and 22c is formed in a region corresponding to the pixel region “P”. A transparent common electrode 24 is formed on the black matrix 21 and the color filter layer 22. A liquid crystal layer 20 is interposed between the reflective electrode 18 and the common electrode 24.
Even though not shown in FIG. 1, a spacer is used to maintain a cell gap between the first and second substrates 6 and 23. A ball spacer having a round shape is generally used. After a fabrication process of the first substrate 6 is finished, the ball spacer is dispersed through a specific method.
FIG. 2 is a schematic cross-sectional view showing a spacer of a liquid crystal display device according to the related art.
In FIG. 2, a spacer 40 is interposed between first and second substrates 6 and 23. A liquid crystal layer 20 wraps the spacer 40. Liquid crystal molecules 20a adjacent to the spacer 40 have a different alignment property from that of liquid crystal molecules 20b remote from the spacer 40 due to an influence of the spacer 40. As a result, light “L” passing through the adjacent liquid crystal molecules 20a causes light leakage in case of a dark state. Moreover, the ball spacer 40 is not uniformly distributed and tends to be concentrated. Sometimes, the ball spacer 40 moves infinitesimally to cause damage to a surface of alignment layers. Further, a very thin cell gap is requested for high-speed response of an LCD device. However, it is difficult to minimize a spacer for the very thin cell gap. Therefore, in order to resolve such problems, a patterning method of a spacer is suggested.
FIG. 3 is a schematic cross-sectional view of a liquid crystal display device including a patterned spacer according to the related art.
In FIG. 3, first and second substrates 50 and 60 face into and are spaced apart from each other. A thin film transistor (TFT) “T” including a gate electrode 52, an active layer 54, and source and drain electrodes 56 and 58 is formed on an inner surface of the first substrate 50. A pixel electrode 59 is connected to the drain electrode 58. A black matrix 62 corresponding to the TFT “T” and a color filter layer 64 corresponding to a pixel region “P” are formed on an inner surface of the second substrate 60. A transparent common electrode 66 is formed on the color filter layer 64.
A patterned spacer 68 having a columnar shape is formed between the pixel electrode 59 and the common electrode 66 through patterning an organic layer (not shown). Although the patterned spacer 68 can be formed on the first substrate 50 or the second substrate 60, the patterned spacer 68 is generally formed on the second substrate 60 (i.e., color filter substrate) because of its flat surface. The patterned spacer 68 may be disposed in a desired region and maintain a firm and stable cell gap by contacting the substrate. Moreover, since the patterned spacer 68 is not formed in the pixel region “P”, light leakage may be prevented. The patterned spacer 68 can be made of a photosensitive organic layer having a negative type or a positive type.
FIGS. 4A and 4B are schematic cross-sectional views showing a fabricating method of a patterned spacer using a negative type photosensitive organic layer according to the related art.
In FIG. 4A, a black matrix 82 is formed on a substrate 80 and a color filter layer 84 including red, green, and blue sub-color filters 84a, 84b, and 84c is formed on the black matrix 82. Each sub-color filter 84a, 84b, and 84c corresponds to a space between the adjacent black matrices 82. A transparent common electrode 86 is formed on the color filter layer 84, and an organic layer 88 is formed on the common electrode 86 through coating a negative type photosensitive organic material (i.e., a negative photoresist). The negative type photosensitive organic material includes solvent, sensitizer, and resin. Generally, the sensitizer initiates a cross-link of the resin by ultra violet (UV) light. The cross-linked resin is insoluble in a developing solution.
A mask “M” including transmissive and shielding portions “C” and “D” is disposed over the organic layer 88. The transmissive portion “C” corresponds to the black matrix 82. After the light is irradiated onto the organic layer 88 through the mask “M”, the organic layer 88 is developed. Since a portion of the organic layer 88 corresponding to the shielding portion “D” is not exposed to the light, the unexposed portion is eliminated and a patterned spacer 90 of a desired shape is obtained, as shown in FIG. 4B.
FIGS. 5A and 5B are schematic cross-sectional views showing a fabricating method of a patterned spacer using a positive type photosensitive organic layer according to the related art.
In FIG. 5A, a black matrix 82 is formed on a substrate 80 and a color filter layer 84 including red, green, and blue sub-color filters 84a, 84b, and 84c is formed on the black matrix 82. Each sub-color filter 84a, 84b, and 84c corresponds to a space between the adjacent black matrices 82. A transparent common electrode 86 is formed on the color filter layer 84 and an organic layer 88 is formed on the common electrode 86 through coating a positive type photosensitive organic material (i.e., a positive photoresist).
A mask “M” including transmissive and shielding portions “C” and “D” is disposed over the organic layer 88. The shielding portion “D” corresponds to the black matrix 82. After the light is irradiated onto the organic layer 88 through the mask “M”, the organic layer 88 is developed. Since a portion of the organic layer 88 corresponding to the shielding portion “D” is not exposed to the light, the unexposed portion remains and a patterned spacer 90 of a desired shape is obtained, as shown in FIG. 5B.
The patterned spacer 90 has different shapes depending on the type of the organic layer 88.
FIGS. 6A and 6B are schematic cross-sectional views showing a shape of a patterned spacer using a positive type photosensitive organic layer according to the related art, and FIGS. 7A and 7B are schematic cross-sectional views showing a shape of a patterned spacer using a negative type photosensitive organic layer according to the related art.
In FIG. 6A, after a positive type photosensitive organic layer 88 (i.e., a positive photoresist) is formed on a substrate 80, a mask “M” including transmissive and shielding portions “C” and “D” is disposed over the photosensitive organic layer 88. When light “L” is irradiated onto the mask “M”, the light passing through the transmissive portion “C” is diffracted at a boundary of the shielding portion “D” toward an inner portion of the shielding portion “D”. Accordingly, a portion of the photosensitive organic layer 88 corresponding to the shielding portion “D” is exposed to the diffracted light. As a result, after the photosensitive organic layer 88 is developed, a patterned spacer 90 of a round shape is obtained, as shown in FIG. 6B.
In FIG. 7A, after a negative type photosensitive organic layer 88 (i.e., a negative photoresist) is formed on a substrate 80, a mask “M” including transmissive and shielding portions “C” and “D” is disposed over the photosensitive organic layer 88. When light “L” is irradiated onto the mask “M”, the light passing through the transmissive portion “C” is diffracted at a boundary of the shielding portion “D” toward an outer portion of the transmissive portion “C”. Accordingly, a portion of the photosensitive organic layer 88 corresponding to the shielding portion “D” is exposed to the diffracted light. As a result, after the photosensitive organic layer 88 is developed, a patterned spacer 90 having a width greater than a desired width is obtained, as shown in FIG. 7B.
The black matrix 21, as shown in FIG. 1, corresponds to the gate line 5, the data line 17, and the TFT “T” in a reflective LCD device. Since the black matrix 21 is designed to include an alignment margin reflecting upon an attachment error of the first and second substrates 6 and 23, the black matrix 21 has an area larger than that of the gate line 5, the data line 17, and the TFT “T”.
FIG. 8 is a schematic cross-sectional view taken along line VIII-VIII of FIG. 1, and FIG. 9 is a schematic magnified view of portion “F” of FIG. 8.
In FIGS. 8 and 9, first and second substrates 6 and 23 face into and are spaced apart from each other. A first insulating layer 10 is formed on an inner surface of the first substrate 6, and a data line 17 is formed on the first insulating layer 10. The data line 17 is disposed between adjacent first and second pixel regions “P1” and “P2”. A thin film transistor (TFT) “T” is also formed on the first substrate 6, and a second insulating layer 16 is formed on the TFT “T” and the data line 17. A reflective electrode 18 is formed on the second insulating layer 16. A black matrix 21 is formed on an inner surface of the second substrate 23, and a color filter layer 22 including red, green, and blue sub-color filters 22a, 22b, and 22c is formed on the black matrix 21. The black matrix 21 corresponds to the data line 17, and each sub-color filter 22a, 22b, and 22c corresponds to the each pixel region “P1” and “P2”.
A patterned spacer 30 having a round shape is formed between the reflective electrode 18 and the common electrode 23. When a distance between the adjacent reflective electrodes 18 over the data line 17 is “a”, the black matrix 21 is formed such that a width of the black matrix 21 is “a+2b”, which is greater than “a”, where “b” is a length of an overlapped portion of the reflective electrode 18 and the black matrix 21. Contrary to a liquid crystal layer 20 over the reflective electrode 18, a uniform electric field is not sufficiently applied to a liquid crystal layer 20 corresponding to “a”. Thus, light can pass through the liquid crystal layer 20 corresponding to “a” even when a voltage for black state is applied to the reflective electrode 18 in a normally white mode. Accordingly, the portion corresponding to “a” should be shielded with the black matrix 21, and the minimum width of the black matrix 21 is “a”. However, since the first and second substrates 6 and 23 are attached with a misalignment, the width of the black matrix should be determined while taking an alignment margin into consideration. Therefore, the width of the black matrix 21 is designed to be “a+2b”, which is greater than “a”. As the width of the black matrix 21 increases, an effective reflection area is reduced. Accordingly, aperture ratio and brightness are also reduced.